Besides Monster Train, what are some of the other Best ones?
Depends on what you're looking for! IIRC you've already played
Inscryption, which would otherwise probably be my #1 recommendation.
I loved
Griftlands, but it's very much narrative first, gameplay second, and didn't have much of a tail for me at all-- I beat it once with each of the 3 heroes and then called it done, since those playthroughs each felt "canonical" to me. But I loved my time with it!
Dicey Dungeons is a delightful game with a pretty long tail. Bright and colourful and funny with lots of distinct tiny challenge runs to mix things up. Has a fair bit of randomness to the gameplay, being built around rolling dice, but lots of tools to smooth out that randomness as you build your kit. It's arguably not quite a deckbuilder, but I'd say it's close enough to count (and one of the classes is outright building a deck of cards).
Roguebook is probably the closest to Slay the Spire, but you have a pair of characters instead of just one, and they each have their own deck. Also, it replaces StS's map (which I didn't like) with a much more robust navigation puzzle when you're revealing the map in segments to try and find battles/events while making a path towards the exit. Oh, and it's all a bit cheerier and more colourful.
Banners of Ruin is another one that I didn't spend a ton of time with, but still quite enjoyed while I was playing it. Its spin on things is that you control a whole party of 3-5(IIRC?), each of whom have their own decks and move around a small grid, and also they're all animal people (sorry, no birbs) and everything is grimdark. Also, instead of a map, you just draw cards from 3 decks, which feels like a more appealing abstraction of what Slay the Spire is doing anyway.
Star Renegades isn't actually a deckbuilder at all, but it lives in an adjacent conceptual space with a lot of the same tensions and decision-making and I enjoyed it quite a lot, so its probably worth calling out (also, technically it has cards for camping, but that's a minor mechanic). You build a party of space rebels and fight an evil empire through time loop shenanigans, unlocking more characters and gear with each successive run, all with a Grandia-style turn system.
Other than that, I've got a few on my backlog that I'm waiting to play, like
Gordian Quest,
Across the Obelisk, and
Nowhere Prophet, all of which I've heard good things about.